This invention relates to a semiconductor layer which has a multilayer structure of alternatingly stacked narrow and wide band gap semiconductor films having a refractive index difference and is for use typically in a surface emitting laser diode.
Surface emitting laser diodes are already known. An example is described in a letter contributed by Y. H. Lee and five others to the Electronic Letters, Volume 25, No. 20 ( 28th September, 1989), pages 1377 and 1378, under the title of "Room-temperature Continuous-wave Vertical-cavity Single-quantum-well Microlaser Diodes". Such a surface emitting laser diode comprises two semiconductor layers of the type described.
In the manner which will later be described more in detail, a conventional semiconductor layer comprises an alternate or cyclic stack of narrow and wide band gap semiconductor films having a refractive index difference and narrower and wider band gaps. The semiconductor layer has a multilayer structure of a plurality of film pairs in this manner and is for reflecting a laser or optical beam of a nominal wavelength. Typically, the narrow and the wide band gap semiconductor films have higher and lower refractive indices of the refractive index difference and may alternatively be called high and low refractive semiconductor films. When only one of the film pairs is taken into consideration, it is possible to refer to the narrow and the wide band gap semiconductor films as first and second semiconductor films.
The surface emitting laser diode of the Lee et al letter is excellently operable. It has, however, been found by the present applicant that the surface emitting laser diode has a high build up voltage at which an electric current begins to flow through the laser diode. This is a serious problem on actually using such surface emitting laser diodes. Furthermore, the laser diode has a high differential resistance value. The high build up voltage and the high differential resistance value are additional serious problems on manufacturing the surface emitting laser diode in an integrated circuit. These problems are unavoidably given rise to by the semiconductor layer which is indispensable in a surface emitting laser diode.